Cameron: Labour has lost its mandate to govern

UK heading for "hung parliament" in which no single party has overall majority

Brown: "My duty is to play my part in Britain having a strong, stable and principled government"

London, England (CNN) -- Conservative Party leader David Cameron said it was "clear that the Labour government has lost its mandate to govern this country," as exit-poll predictions put his party on course to win more seats than it had for 80 years.

He said the Conservatives had fought a "positive and energetic" campaign. It was clear from the results that "the country wants change" and "that change is going to require new leadership."

Exit polls suggest the Conservatives are on pace to win 305 seats -- though this would be 21 short of a majority in the 650-seat House of Commons.

Prime Minister Gordon Brown gave no indication early Friday that he would step down after being returned to parliament by his constituents in Kirkcaldy, Scotland.

The "outcome is not yet known but my duty .. is to play my part in Britain having a strong, stable and principled government," he said. "I will not let you down.

"I am very determined and have been through quite a lot in my political career and in my personal life, and I am used to difficulties."

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If the predictions are borne out by results, the UK is heading for a "hung parliament" in which no single party controls an overall majority.

The leader of the largest party traditionally gets the first chance to form the government and become prime minister. But if no party has a majority, the sitting prime minister has the right to stay in office and try to win a confidence motion in parliament.

"The sitting prime minister and the incumbent government are given the first chance to create a majority that commands the confidence of the House of Commons, and if they fail to do that it passes to the leader of the opposition," top Labour politician Peter Mandelson told CNN.

Brown arrived at Labour Party headquarters in London early Friday after flying in from Scotland. He smiled and shook hands as he entered the building with his wife, Sarah, but said nothing to waiting cameras and reporters.

Several high-profile incumbents lost their seats overnight, including former Home Secretary Jacqui Smith, of Labour; Northern Ireland First Minister Peter Robinson, leader of the Democratic Unionist party; and Liberal Democrat Lembit Opik, who had been in parliament for 13 years.

All had been implicated in last year's parliamentary expenses scandal. Robinson's loss could hurt the Conservatives, robbing them of a potential supporter in the Commons should the party need to form a coalition.

The Green Party, which favors environmental and social justice policies, gained its first-ever member of parliament when Caroline Lucas was elected in Brighton, in southern England.

There were some scenes of voter anger across the country over long lines to cast ballots or polling stations running out of ballot papers, but it was not immediately clear how widespread problems were.

"The country has voted for change and the idea that Gordon Brown can hang onto power I think most people would find staggering.--George Osborne

"We will be doing a serious and thorough review of this and making recommendations to parliament and the government," Electoral Commission Chairwoman Jenny Watson said.

The United Kingdom's system of voting is "Victorian, antiquated, left over from an era when less people had to vote" and not designed to cope with mass participation, she said, adding that the system is now "at breaking point."

It's very unusual for no party to get an absolute majority of seats in the Commons. The last time it happened, in 1974, voters were back at the polls within months.

After the election there will be 650 seats in the Commons, four more than in the previous parliament. Voters chose representatives for only 649 seats, however, because the death of a candidate in northern England postponed that election to May 27, local officials said.

Under the British electoral system, the candidate who receives the most votes in a constituency wins. The system, known as "first past the post," is praised for its simplicity and the strong ties it forms between voters and representatives, but critics dislike its failure to provide proportional representation.

CNN's Zain Verjee, Richard Greene and Paul Armstrong in London contributed to this report.